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Aviation History
1934
1934 - 1392.PDF
SUPPLEMENT TO FLIGHT THE AIRCRAFT ENGINEER JANUARY 25, Sparking Plugs There 13 no more difficulty in obtaining a suitable plug tor operation with leaded fuel than choosing one for any engine whatever the fuel used. In either case a satisfactory choice largely resolves itself into a matter of practical' test. The three troubles which may pos- sibly occur are (1) an excessive rate of build up of de- posit on the insulator; (2) possible breakdown of the (/HMODIfiED PLUQ Of HOT 6AS. So*ie Gerosir mu. ftHTM He#E trs . v 6000 aezc.- TMEVE M*Y SM I BUILO IM Of O**osrr\ - O>t/s//iG Ore* He*Tt*e //y ThisOue Tb /rsS—*LL Fig. 10—This sketch shows moditications made to mica sparking plugs to suit air-cooled engines using a "leaded" fuel. insulation; and (3) erosion or corrosion of the electrode tip. These are not necessarily related, and are dif- ferently influenced by temperature. More complete de- tails will be found in the main text, but let it be said that any troubles experienced with sparking plugs are not critical, and the cures are relatively straightforward. Some Notes on " Stelliting " Although in use for some time, the application of Satellite to the valve and seat inserts of aviation engines is only a relatively recent development, consequently the following information may be of some assistance. " Stellite " is a material composed largely of cobalt and chromium, between 50-65 per cent, of the former and 30 per cent, of the latter, together with tungsten, which may vary, in proportion, from about 4 to 20 per Fig. 11—This diagram shows the various stages of stelliting a valve seat. cent. The amounts of the constituents vary according to the particular grade and degree of hardness required. Both at low and elevated temperatures its resistance to wear and oxidation is excellent. It is particularly effec- tive in resisting, at high temperatures, corrosion attack by lead bromide or the products of its decomposition. The treatment of valves and inserts consists, briefly, of applying a layer of " Stellite " to the seat surfaces, this being done by means of an oxy-acetylene torch. " Stelliting " is not, actually, a welding process, since fusion of the Stellite and the material of the part being treated, is not desired. It might be described as a high-temperature brazing process. A description of the technique of applying Stellite will be found in the main text. The figures 11 to 15, however, show the essential points in the process. Fig. 12 shows a typical example of an exhaust valve of a large American radial engine. The valve has a hollow forged head, sodium filled, and both the seating and valve head are Stellited. This valve measures about 3 in. across the outer diameter of the seating and weighs approximately one pound in its completed form. Fig. 13 gives a sectional view of the same valve. Note the hollow swaged in plug at the stem end, and also the thickening up of the stem section at the neck. This valve is a remarkable example of accurate forging. The illustration in Fig. 14 shows Stellite deposited on a rough machined valve of martensitic steel. The segment cut from the valve head shows up the section. Fig. 15 shows a valve of the same type as Fig. 14, but it is of austenitic steel similar to K.E. 965, and is the pro- duct of another manufacturer. Note the difference in the Stelliting. (To be continued.) Figs. 12 to 15—Show further details of different valve types and methods of " stelliting." ;' "" 78 A
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